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Transport Model Sensitivity for Soil Cleanup Level Determinations Using SESOIL and AT123D in Context of the California Leaking Underground Fuel Tank Field Manual

The California Leaking Underground Fuel Tank Field Manual (LUFT Manual; WRCB, 1989) is used by the regulatory community, consultants, and industry in California to determine acceptable cleanup concentration goals for the remediation of hydrocarbon-affected soils. The LUFT methodology is a semi quantitative approach that uses rating tables that consider the effects of local precipitation and the depth to ground water from the deepest affected soils, as well as anthropogenic and geologic factors. The latter factors are evaluated subjectively, with only the effects of local precipitation and depth to ground water accounted for quantitatively. To assess the effects of these variables on the hydrocarbon concentrations that could be left in soil while protecting ground water quality, the state of California performed modeling using SESOIL and AT123D. The results from a small number of simulations covering a very narrow range of input parameter values were then extrapolated to form the LUFT Manual rating tables, which cover ranges in precipitation and depth to ground water of 0 to 40 in. per year and 5 to 150 ft., respectively. Although the use of these tables generally results in conservative cleanup level determinations, the extrapolation method used and the lack of consideration for extremely sensitive input parameters (other than precipitation and depth to ground water) in the development of the tables calls into question their validity. A sensitivity analysis on the model input parameters is presented that highlights several critical input parameters that greatly affect cleanup concentration determinations. The sensitivity analysis shows that certain parameters that were fixed a conservative levels for the development of the LUFT Manual rating tables (e.g., biodegradation rate and soil organic carbon content) are more sensitive than precipitation and the depth to ground water. In many cases, site-specific analysis will thus yield higher soil cleanup concentrations that are still protective of water quality. In addition, in some instances the cleanup concentrations in the LUFT Manual tables are not protective of water quality. To provide a firm basis for soil cleanup-level determinations, Site-specific analysis is recommended whenever significant quantities of soil may require remediation. This will provide more cost-effective remediation and greater assurance of water quality protection.